Andrew V. Pakhomov

Ablative Laser Propulsion: Specific Impulse and Thrust Derived from Force Measurements

Specie c impulse, thrust, and other dynamic characteristics of solid elemental propellants for ablative laser propulsion are studied experimentally with the aid of a piezoelectric force sensor. This sensor allows direct measurements of applied thrust. Experimental results show that the highest specie c impulse, » 4:0 ££ 10 3 s, occurs for carbonand aluminum, whereasthehighestcoupling coefe cient (8dyne/W)wasobserved forleadtargets.Theablation time,derived from independenttime-of-e ight (TOF)experimentsand used forthecalculation of impulses, was » 1.5 πs for the majority of the elements studied. These new measurements cone rm a decrease of specie c impulse and an increase in thrust with increasing atomic mass of the propellant; the major trend previously determined from TOF experiments. The plasma image analysis also cone rms the previously reported angular independence of ion velocity, established after the e rst 100 ns of plasma life. In previously reported TOF studies, all observations were based on the ionic component ejected from a laser-ablated material. An almost e vefold reduction in the absolute values of the specie c impulse was observed in the current effort. This result is attributed to “ dilution” of the high specie c impulse values for ions by slower atomic neutrals and larger fragments. Among all of the tested elements, aluminum is found to provide the optimum tradeoff for Isp and thrust values.

Laser-induced phase explosions in lead, tin and other elements: microsecond regime and UV-emission

Phase explosions (PEs) have been observed at tens of microseconds delay after laser pulse arrival on carbon, aluminium, silicon, iron, copper, zinc, tin and lead targets ablated at an irradiance ~3×1013 W cm−2, delivered by 100 ps laser pulses at 532 nm wavelength. The PEs in lead and tin were accompanied by an emission that caused a massive photoelectric effect from copper plates that were positioned 20 and 40 cm away from the targets. Initial estimations suggest emitted photon energy around 10 eV. The phenomenon was also studied by direct imaging with an intensified charge-coupled device camera. Analysis of the PE event is given in terms of thresholds, critical exposures (i.e. number of laser shots required to cease observation of PE), angular profiles of UV-emission and ejected particulate velocities. The velocity of the ejected particulates was ~200 m s−1, which is about 100 times slower than the ion velocity. The fraction of mass removed via PE was ~90%. The observations and data analysis reported in this paper can hardly be explained without considering the phase boundaries and/or charge transport. This poses certain challenge to current thermal models of PEs.

Ablative Laser Propulsion: A Study of Specific Impulse, Thrust and Efficiency

This paper represents a review of studies devoted to Ablative Laser Propulsion and conducted by the Laser Propulsion Group (LPG) at the University of Alabama in Huntsville.

Specific Impulse and Other Characteristics of Elementary Propellants for Ablative Laser Propulsion

This work was performed as a continuation of ablative laser propulsion (ALP) research recently revived by the authors. An ALP-based space vehicle would be driven by high-energy, short-duration (10 -10 s and less) laser pulses, focused on a solid propellant. Under such irradiation conditions, plasma and laser pulses are temporally separated, and the direct ablation of solid propellant dominates other possible momentum transfer mechanisms. The studies were performed for various elemental targets, ablated at irradiances up to ∼3 × 10 13 W/cm 2 delivered by 100-ps laser pulses at a wavelength of 532 nm. The data, collected predominantly from time-of-flight technique, include ion velocities, number densities, ion density angular distributions, and mass-removal rates. From these data, specific impulses and net momenta exerted on targets by ablation were calculated. For the irradiation conditions used, the highest I sp and lowest mass-removal rates were achieved for low-Z targets, whereas high-Z elements provided higher momenta. The upper limits for I sp derived from ion velocity reached 2 x 10 4 s with carbon (graphite). Ion energies varied from element to element in the range of ∼0.4-2.0 keV, with ion fractions of ∼0.8-3.8% and mass-removal rates in the range of 0.1-3.0 μg per laser pulse. Ion velocities exhibit Maxwellian distributions, whereas ion number densities in the plasma plume were fitted to a cosine function. All studied characteristics indicate that high-I sp ALP must be based on solid low-Z propellants such as a carbon or aluminum.

Air pressure effect on propulsion with transversly excited atmospheric CO2 laser

The assessment of energy partition between air and solid propellant has been conducted using a transversely excited atmospheric CO2 laser. The experiments were performed by focusing output pulses of the laser (200-ns pulsewidth at 10.6-µ mw avelength and ∼10.6-J pulse energy) on aluminum targets mounted on a ballistic pendulum. Coupling coefficients and mass removal rates were determined as functions of air pressure, which varied from 1 atm to 3.5 mtorr. The data from both coupling coefficients and mass removal rates show that there is a sharp transition region ranging between 1.0 and 10 torr. In this region, the momentum imparted to the target via air breakdown appears comparable to the momentum due to the breakdown on the target surface. At pressures exceeding 10 torr, the coupling to the target due to air breakdown dominates the ablation.

Laser Propulsion with Liquid Propellants Part I: an Overview

Despite years of research, laser propulsion on liquid propellants has yet to achieve specific impulses greater than tens of seconds. It is well established that liquids, when used as propellants, can provide coupling coefficient (Cm) on the order of 100–1000 dyne/W. However, the specific impulse (Isp) has proven to be significantly inferior to that for solid propellants. This paper will examine the various laser propulsion schemes based on ablation of liquid propellants and intended primarily for space applications. The principal shortcomings associated with liquid propellants will be outlined.

Laser Propulsion with Liquid Propellants Part II: Thin Films

Thin films of a liquid propellant have been studied as a potential way to boost thrust for laser propulsion applications. A TEA CO2 laser with 300 ns pulse width was operated at up to 20 J pulse energy to produce irradiances at the target on the order of 1–1200 MW/cm2 to ablate various systems of thin films on Delrin® substrates. In this study, time‐resolved force sensors and ICCD imaging techniques were used to determine how an addition of thin liquid films to solid substrates affects propulsive properties such as momentum coupling coefficient, specific impulse, and internal efficiency. Transparent (hexane) and absorbing (ethanol and water) thin films were formed above Delrin® substrates for the laser system operating at 10.6 μm. Thickness effects on the hexane‐Delrin® system will be examined. An analysis will be made of the possible routes for force generation, and the general properties, benefits, and shortcomings of liquid thin film structures will be summarized with regard to laser propulsion.

Experimental Study of Coupling Coefficients for Propulsion on TEA CO2 Laser

The original purpose of this study was to address a partition of propulsive energy between air and metal, when the breakdown is initiated at the metal surface and/or in adjacent air space. Coupling coefficient as a function of air pressure varied in the range 4 mTorr – 1 atm is presented. The experiments were conducted by focusing output pulses of a TEA CO2 laser system (0.2‐μs pulsewidth at 10.6 μm wavelength and ∼ 10.0 J energy) on aluminum targets. Coupling coefficients were derived from the pendulum displacements.

Ablative laser propulsion: determination of specific impulse from plasma imaging

This work summarizes the combination of experimental and digital image processing technique developed for determination of plasma expansion velocity angular profiles. Such profiles were used further for assessment of specific impulses for ablative laser propulsion. The technique uses time-resolved intensified charge-coupled device (ICCD) camera with 18 ns minimum time delay, 100 μm spatial resolution, and 5 ns gating speed. The plasma was formed in vacuum (~ 3x10-3 Torr) by focusing output pulses of a laser system (100-ps pulsewidth at 532 nm wavelength and ~35 mJ energy) on surfaces of C (graphite), Al, Si, Fe, Cu, Zn, Sn, and Pb targets. Plasma expansion velocity profiles were derived from plume edge contours. Specific impulse (Isp) was then deduced from the profiles. New Isp data appeared in excellent agreement with specific impulses derived from force measurements, conducted earlier. Observed angular profiles of plasma edge velocity and integral intensity are reported and discussed.

Laser-Driven Mini-Thrusters

Laser‐driven mini‐thrusters were studied using Delrin® and PVC (Delrin® is a registered trademark of DuPont) as propellants. TEA CO2 laser (λ = 10.6 μm) was used as a driving laser. Coupling coefficients were deduced from two independent techniques: force‐time curves measured with a piezoelectric sensor and ballistic pendulum. Time‐resolved ICCD images of the expanding plasma and combustion products were analyzed in order to determine the main process that generates the thrust. The measurements were also performed in a nitrogen atmosphere in order to test the combustion effects on thrust. A pinhole transmission experiment was performed for the study of the cut‐off time when the ablation/air breakdown plasma becomes opaque to the incoming laser pulse.